A model to evaluate the consequences of GM and non-GM segregation scenarios on GM crop placement in the landscape and cross-pollination risk management

Under European regulations, a product is labelled as GM (genetically modified) if more than 0.9% of one of its ingredients originates from GM material. During collection, crops from many fields are combined to fill a silo. To avoid the risk of mixing GM and non-GM harvests, it is possible to dedicate a silo to a given crop or to define specific times for GM and non-GM product delivery to silos. To evaluate these scenarios for the maize supply chain, we propose a combination of a model of farmers' varietal choice (based on profit evaluation at the field level, taking into account transport costs as well as price and cost differences between GM and non-GM products) and a spatially-explicit gene flow model. Consequences of different segregation strategies for collection zone organization can therefore be compared while using the percentage of GM grain in non-GM crops due to cross-pollination. The 'temporal' strategy leads to a uniform area of GM or non-GM maize, depending on the prices and the weather risks. The 'spatial' strategy leads to areas of either GM or non-GM crops surrounding the corresponding collection silo. GM presence in non-GM batches depends on the size of the non-GM zone and on the prevailing wind. We show how divergent commercial strategies of grain merchants could have consequences on GM presence in non-GM batches.